Analysis of group 3 cations

Chromium confirmatory tests

We saw before that the treatment with NaOH and hydrogen peroxide should have highlighted the presence of chromium. There are few other chemical species that color in yellow the solution. However, for further confirmation we can perform a couple of tests.  Actually, we already know the first one from group 1 analysis.

 

1) treatment with lead acetate (CH3COO)2Pb

This test follows exactly one of  the identification essays we saw in group 1 in order to confirmate the presence of lead in the initial substance.

In this case also we need to create a slightly acidic environment. Working this way we do not precipitate lead (Pb2+) as lead hydroxide((Pb(OH)2)) and we do not transform the chromate in dichromate. Using acetic acid buffer we fix the pH around 5 (verify with litmus test).We can confirm the nature of the precipitate checking the solubility in strong bases (the precipitate dissolves because lead is moved to plumbate). For more details → lead confirmation.

2) Chromium (VI) oxide peroxide

It's a very particular and specific essay to identify chromium compounds.

The solution is alkaline because of soda (see above). Through addition of nitric or sulfuric acid the pH is brought around 3. Subsequently we add a few mL of diethyl ether or isoamyl alcohol to the test tube (enough to form a two-phases system) and then a few drops of hydrogen peroxide. Plugging with the finger we shake the test tube to extract the new compound (chromium oxide peroxide) in the organic layer. The test is extremely sensitive but requires to be performed correctly. The pH is fundamental and, it has to be done without heating and very quickly. Chromium oxide peroxide is indeed very unstable, and could decompose in the acqueous phase before we start shaking.

CrO42− + 2 H2O2 + 2 H+ → CrO5 + 3H2O
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